Breunnerite, also known as brown spar () is a variety of magnesite, with a magnesium/iron ratio of 90/10 to 70/30.
It has been described by Wilhelm Karl Ritter von Haidinger in samples of Pfitsch pass, Zamsergrund and , two cities of the Ziller Valley, Tyrol, Austria.
Haidinger named the new variety of the magnesite in honor of Count (sometimes the family name is written as Breuner; 1796-1877), a famous collector of minerals and fossils, as well as a high-ranking government official of Austria-Hungary.
In 19th-century mineralogy, as well as in mining and among geologists, breunnerite and its close analogues from the isomorphic series magnesite â siderite were more often known under the capacious morphological name â³brown sparâ³.
Breunerite is widely used in firing during the formation of raw materials for the subsequent production of refractories. When magnesium carbonate, which is part of breunerite, is brought to a temperature of about 600ðC, it enters the firing process as a mineralizer, first decomposing and then reacting and forming two main compounds that act as raw materials: 2CaFe<sub>2</sub>O<sub>3</sub> ø MgOFe<sub>2</sub>O<sub>3</sub>. Breunerite produces a less pure product than a mixture of magnesia and iron oxide, but is more economical to produce.